1. Field of the Invention
This invention relates to fusible resistors.
2. Prior Art
In certain electrical and electronic circuits the failure of one component can cause a catastrophic failure of one or more other components. For example, an excessive current in a transistor in one part of the circuit can cause an excessive current to flow in one or more other parts of the circuit with the result that semiconductor devices suffer irreversible breakdown. Any protection device has to operate at a current level lower than the critical level and within a specified time and such a protective device has to either limit the current or cut it off. In order to cut off the current a conductive path has to be broken and this of course is the traditional function of a fuse.
As is known the effect of an excessive current (assuming a constant voltage) is to increase the heat dissipation and protective resistors using this effect have already been used. For example it is known to provide a protective resistor having a spring clip through which the current flows and which has one end soldered to the body of the resistor. When the heat dissipation in the resistor exceeds a critical value the temperature of the resistor rises above the melting point of the solder and the spring clip is released creating a break in the conducting path whereupon current ceases to flow.
It has also been proposed to provide a protective device in the form of a thick film resistor. In this proposal there is first screened on to a ceramic substrate two strips of relatively conductive material which strips can be considered as terminal strips. The substrate carrying the two terminal strips is then baked and fired at a high temperature whereafter relatively resistive material is screened on to the substrate so as to extend between the terminal strips and then the substrate with the resistive material on it is baked and fired.
In certain cases instead of screening a resistive material onto the substrate so as to extend between the terminal strips, the conductive material is screened on to the substrate so as to form a relatively long path between the terminal strips to provide a low value resistor; for this purpose the path of the conductive material between the terminal strips may be that of a zig-zag or any other meander path. Connections to the terminal strips are usually made by press-fitting tags onto the substrate which tags are then soldered to the conductive strips. In this way a thick film resistor is formed and the wattage rating of this resistor will depend on the area and thickness of the substrate, the amount of heat lost, in use, by conduction through the tags to, for example, printed circuit boards, and heat radiation and convection. When, however, the rated wattage is exceeded by a certain factor, the substrate is stressed by thermal expansion to the extent that it breaks. The thickness of the substrate is one of the factors controlling the time and wattage at which the break will occur.